Orthodox Jewish children wearing Star of David patches and uniforms similar to those the Nazis forced Jews to wear, attended a rally in December 2011. Many Orthodox Jews in Israel see a growing threat to their way of life.

When Tanya Rosenblit boarded the No. 451 bus to Jerusalem last month, she knew that the predominantly ultra-Orthodox passengers would keep their distance from her because of their adherence to strict rules of gender segregation.

But when one of them demanded she move to the rear, Ms. Rosenblit held her ground.

"He said, 'Respect me by moving to the back of the bus,' " she said. "I said he should be ashamed he didn't respect [his] own mother or daughter. I asked him why it was so horrible for a woman to sit at the front of the bus. I was really angry."

Rosenblit's Rosa Parks-like confrontation helped reveal the fault line between mainstream Israel and the insular ultra-Orthodox, or Haredi, minority in a way not seen in years.

Culture clashes over Sabbath observance rules have been commonplace for decades. But reports of the creeping exclusion of women from public places has touched a raw nerve in the Jewish state by raising the prospect of an Israel dominated by a religious extremism similar to hard-line ideologies prevalent in neighboring Muslim states.

It also has added to fears that Israel's much-touted democracy and its underpinnings of individual rights and equality are under threat.

"This is an important moment for Israel. The good news is that both the public and the political system are responding with the appropriate outrage," says Yossi Klein Halevi, a fellow at the Shalom Hartman Institute in Jerusalem.

"One of the responsibilities that have been forced on the state of Israel because of its regional location, and its location in the Holy Land, is to be a force against religious fundamentalism. That's true in Israel's frontline role against jihadism, and it's true as well for maintaining an open society in one of the most religiously intense countries in the world."

The culture clash is being heightened by some stark demographic trends: Thanks to high birthrates, the percentage of ultra-Orthodox in the adult Israeli population has doubled to 8 percent over the past 20 years. In another 50 years, they will make up nearly a third of the population, according to government statistics.

Along with the newfound political and economic clout, ultra-Orthodox Jews are gradually moving from the margins of society into mainstream jobs, neighborhoods, and organizations like the army.

Recognizing their leverage, companies and government agencies have quietly deferred to their requests for gender-segregated buses and health clinic waiting rooms, and for billboards cleansed of female images.

To be sure, many ultra-Orthodox see themselves as a minority persecuted by Israel's secular mainstream, which wants to intervene in a subculture that has long tried to protect its values through bans on movies, television, and the Internet.

"It is important that the secular society respect their way of life," says Sari Rot, a female reporter for ultra-Orthodox news website Behadrei Haredim. "I wouldn't expect that someone would force me to shake the hand of man."

Some Haredi demands, such as excluding female soldiers from singing at military ceremonies, flew under the radar of most Israelis. But the cases of Rosenblit and – a week later – Naama Margolis, an 8-year-old girl who was called a prostitute and spat on for not dressing modestly enough, galvanized the Israeli public.

"It takes a lot to shock Israelis, because they've seen so much here. They don't have time to think about other things," says Orly Erez Lihovsky, a lawyer for the Israel Religious Action Center. "It's at a stage where it can't be ignored anymore."

The issue has also resonated with Orthodox Jews. Shaltiel Adani, whose Orthodox upbringing included activities involving men and women together but prayer services that were conducted with separate seating, says Haredi follow the most stringent adherence to Jewish law. That, says Mr. Adani, leads Haredi sects to compete to be most hard-line.

"It's Khomeinism," he says, referring to the leader of Iran's 1979 revolution. "Whether it's Judaism, Christianity, or Islam, people are going more toward religious [extremism] ... [Israel] already has a bad image, and this simply pours oil on the fire. It doesn't matter that it's a small extreme ... the world doesn't see it that way."

Birthrate twice the national average

As an insular community of Jews who adhere to a religious ideology that rejected the basic tenets of modern Zionism, ultra-Orthodox have always been a quaint oddity to mainstream Israelis.

Israel's founding generation was content to ignore the Haredim as a vestige of Europe doomed to vanish, giving them military draft exemptions and autonomy over publicly funded schools in return for political patronage. And so the Haredim established ghettos in Jerusalem and the Tel Aviv suburb of Bnei Brak, avoiding the army for fear of assimilation and the workforce in favor of religious studies.

But instead of vanishing, the communities have outpaced the growth of all other Israeli groups because of a birthrate more than twice the Israeli average. Israelis are now increasingly worried about the challenges of integrating a group with low labor force participation and increasingly strict religious ritual dictated by rabbis who are often at odds with the state.

About 60 percent of Haredi men don't work, pushing Israel toward "a third-world economy," says Shahar Ilan, vice president of Hiddush, an Israeli nonprofit that promotes religious freedom. "There is a fanatical tendency in the Haredi community, which is explained by the fact that modern life threatens them," says Mr. Ilan.

Closing ranks around extremists?

Many assert that the majority of Haredim oppose the extremism but are either afraid to speak out or are closing ranks, seeing social criticism as part of a broader discrimination.

"The Haredi society for the most part doesn't identify with the actions of extremists," says Ms. Rot. But there is broader support for measures like gender segregation as a protection of religious values.

Yisrael Eichler, a parliamentarian from the ultra-Orthodox United Torah Judaism party, says "The Haredi public is very polite and doesn't make provocations" and says that the segregated bus lines – known as mehadrin, or kosher – were created to eliminate crowding and shoving between men and women.

"It is very uncomfortable for men and women because in the Haredi community there is no contact between men and women except in the family," he says.

On Egged 292, a mehadrin bus line through the ultra-Orthodox city of Bnei Brak, a grandmother explains that Haredi women such as herself usually sit at the rear voluntarily so as not to come into unwanted contact with men.

"What did you expect, a cage? This is what is accepted. Some sit in front and some sit in the back. No one is shouting and no one is spitting," she says. "I feel wonderful."

Rosenblit, for her part, says she has yet to take the same bus line to Jerusalem and says she has received death threats.

Despite the hostility, she also sees that the ultra-Orthodox fear the Israeli mainstream and says she respects women who believe in gender segregation.

"Everything has a nice explanation, but on the surface of it, they just don't understand the consequences – which is lack of freedom," she says.

But if the mainstream acts too forcefully, the conflict could escalate. "In a way their fear of accepting something different from them is their problem, and our lack of understanding is part of the problem. We fear them as much as they fear us."